Although siblings of children with autism have been found to be at elevated risk for social and communication impairments, research has not yet focused on the very early development of these children. The primary advantage of studying young siblings of children with autism is the opportunity it affords for the identification and remediation of early deficits that could have adverse effects on subsequent development. Deficits in social-communicative behaviors are the most prominent early symptoms in children with autism, and have recently been hypothesized to indicate the presence of a fundamental, autism-specific "social orienting" impairment. The proposed project will study the early development of four dimensions of social orienting (i.e., initiating affect, initiating attention, responding to affect, responding to attention). In typically developing infants, these classes of behavior appear quite early and set the stage for the emergence of important developmental accomplishments such as establishing social-emotional reciprocity and using referential language. Social orienting behaviors will be examined longitudinally in two sibling groups: younger siblings of children with autism spectrum disorders (SIB-ASD) and younger siblings of typically-developing children (SIB-TD) who are CA-matched. Children in both groups will enter the study between 12 and 18 months of age and will be followed for two years (i.e., through ages 36-42 months). Social orienting behaviors will be measured at six time points over the 2-year period, and social and language outcomes will be measured at the end point. Additionally, two age-matched comparison samples will be recruited to provide end-point references for social orienting behavior: children with ASD and children with nonautistic developmental delays (DD). The inclusion of these samples will enable us to compare social orienting at age 3 in siblings of children with ASD with that of: a) children with typical development; b) children with ASD; and c) children with DD. This project represents a unique contribution to the autism literature in its focus on very young siblings of children with autism; its systematic examination of different components of social orienting; its longitudinal design; and its use of ASD and DD cross-sectional samples at age 3 to provide benchmarks for delay and deviance in social orienting skills.